If your needs are that simple:
My Ubuntu 12.04 remix will fly on that machine. Its actually what I use on my G5 (with void on another partition for testing purposes). Super easy install. The dvd boots to a live desktop to test before installing.
2023 update For anybody still using this (16.04 v2) i created a tutorial with some extra files to extend the life of it. Basically we change the sources.list to Debians (since Ubuntu's repo is dead) and make a few changes to allow packages to be updated and installable again. Download it here...
forums.macrumors.com
Linked near the bottom of first post.
Otherwise there's still Ubuntu 16.04, Adelie, Void-PPC, MintPPC (debian based), Fienix (debian based), Crux-ppc, and Gentoo. There's also the BSD's but they have crap for browsers.
As for youtube....
I came across this, and decided to give it a shot on Ubuntu 16.04.7 (remix). I'd assume it'd be possible to build the CLI version on 10.4/10.5 as well with macports perl, but i haven't tried that yet. It works very well. Fast too. Download the 0.0.8 release found here. Now you'll need to...
forums.macrumors.com
...and updates for the included browsers with a few extra browsers added: (scroll the whole page for more)
Web Browser for PowerPC Linux and Mac OS X 10.6 and above - wicknix/SpiderWeb
github.com
Cheers